Low energy electron diffraction using an electronic delay-line detector

Abstract

A low energy electron diffraction (LEED) instrument incorporating a delay line detector has been constructed to rapidly collect high-quality digital LEED images with low total electron exposures. The system uses a position-sensitive pulse-counting detector with high bias current microchannel plates. This delay-line detector combined with a femtoampere electron gun offers a wide range of flexibility, with electron dosing currents ranging from 0.15pAto0.3fA. Using the highest current setting and collecting 1×106 counts per image, individual LEED images can be completed in 4s with an acquisition rate of 250kHz and a total electron exposure of 5×106 electrons. Under the latter conditions, images can be collected in 20min with an acquisition rate of 1kHz with a total electron exposure of 2×106 electrons. An angular width of 0.13° at 108eV is demonstrated, which means that domain sizes as large as 600Å can be resolved, depending on the surface quality of the crystal. The system electronics collect 2048×2048pixel images with a spatial resolution of about 75μm. The dynamic range of this system is 32bits∕pixel (limited only by physical memory). The construction of the detector results in a “plus”-shaped artifact, which requires that, for a given sample orientation, two images be taken at a relative angle of 45°. Identical current-voltage curves from an MgO(111)1×1H terminated sample, taken during several hours of exposure to the low current electron beam, demonstrate minimal electron induced H desorption.